81 • REPOKT 1876. 



in P. frontalis it is covered, and in P. verrecnuvii it is enclosed ; and Milne- 

 Edwards observes that many other Crustacea offer examples of these three 

 organic forms. For instance Pagunis ccenohites and Calianassa have the 

 ophthahnic somite exposed as in. Palinwus vulgaris; Honmrus, Crangon, Pa- 

 Icemon, Galathea, Lithodes, lianina, &c. have this somite covered as in 

 Palinur us frontalis; and Homola has the ophthalmic somite enclosed. 



In Astacus the ophthalmic somite is reduced to a minimum extent, and it 

 is only partially protected by the anterior projection of the rostrum of the 

 carapace. 



Milne-Edwards says that, independently of the somite, the ocular appen- 

 dages are formed of three " articles " or joints, a eoxophthalmite, a basoph- 

 thalmite, and a podophthalmite, but that ordinarily the eoxophthalmite is 

 rudimentary or obsolete. 



In the genus Alplueus (fig. 10) and other fossorial marine forms the ocular 

 appendage is reduced to an extent that allows the carapace to cover it 

 entirely; but in the larval form the organ (fig. 11) is seen to be as well de- 

 veloped and as prominent as that of any aquatic species. It is in this way 

 we may assume that the sessile condition of the organ in the Edriophthalmia 

 (fig. 12) has been attained, first by the contractiou or redaction in extent of 

 the ocular appendage, so that the anterior wall of the carapace shall cover it, 

 and then by the more intimate connexion of the organs with the structure of 

 the parts that protect them, and ultimately with entire absorption of the 

 ocular appendage ; the eye receives its support from the walls of the cara]3ace 

 alone. 



Even here the organs are themselves still liable to depreciation ; thus 

 those that exist where light is absent (which inhabit decj) wells, subter- 

 ranean caves, and excavations in the depths of the ocean) first lose the 

 dark colour of the reflecting pigments, which is soon followed by a degene- 

 ration of the character and appearance of the lens. In Amjpelisca, an Am- 

 phipod that lives in muddj- bottoms, all the lenses but two have disappeared, 

 and the pigment has become red ; in the well-shrimp {Niphargus) the only 

 trace of an eye exists in some yellow-looking pigment; while in the Podo- 

 phthalmia we find that Polycheles (Heller), a jn-awn from the Adriatic closely 

 allied to (if not identical with) Didamia from the deep-sea dredging of the 

 ' Challenger ' expedition, and another from the Mammoth Caves of America, 

 as well as Nephrons Stewarti (Wood-Mason) from Formosa, have the eyes 

 wanting as organs of vision, while they retain them as obsolete appendages. 



The second pair of appendages is the first pair of antenna?. These M. 

 Milne-Edwards has named (for the sake of convenience in distinguishing them 

 from the second pair) the antenmdes. Eut as this term is one, in itself, 

 that is suggestive of dimiuutiveness and inferiority, I think that it had better 

 be employed as little as possible. Generally speaking, this pair is smaller in 

 proportion than the second ; but usually it is of a more highly organized 

 structure, and diminishes in dimensions as it becomes important in its 

 functional properties. 



The appendage consists, in its normal condition, of three joints, homo- 

 typical of the coxa, the basos, and ischium of the true legs in Crustacea. 

 These three joints support an extremity that is very liable to vary in form, 

 number of branches, and general appearance ; but one of them must be re- 

 garded as the primary branch, iuasmucli as it is invariably furnished with a 

 set of organs peculiar to it, and found on no other part of the animal. 

 These are slender, delicate, mcra1n-anous, thread-like processes, that are 

 liable to vary somewhat in form and size, but are all but universally present 



